The performance of many devices and manufactures comprised of multiple layer thin-film structures can be significantly improved by increasing the strength of a preferred crystalline growth orientation of at least an active layer of the multiple layer structure, e.g., a preferred hexagonal close-packed (hcp) <0002> or face-centered cubic (fcc) <111> out-of-plane growth orientation of the active layer. For example, when fabricating high areal recording density perpendicular magnetic recording media (described in detail below), it is frequently desirable to deposit well-oriented, i.e., out-of-plane oriented, (hcp) <0002> and (fcc) <111> layers in the same thin-film structure.
According to conventional practice, an upper layer is grown in an oriented fashion on a lower layer by a coherent growth mechanism. Unfortunately, however, coherent growth of the upper layer on the lower layer requires the former layer to be chemically compatible with the latter layer, with substantially similar crystal lattice structure and atomic interplanar spacings (lattice parameters), since materials with sufficiently different crystal structures and lattice parameters do not grow coherently one upon-the-other, disadvantageously resulting in loss of preferred growth orientation. Specifically, loss of a desired preferred crystallographic growth orientation of the active magnetic recording layer of a perpendicular magnetic recording medium often results in undesired, deleterious change in the crystal grain structure and size, in turn manifesting in reduced or degraded media performance characteristics, e.g., magnetic anisotropy (Ku) and coercivity (Hc).
The above-mentioned magnetic media are widely used in various applications, particularly in the computer industry for data/information storage and retrieval, and efforts are continually made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. Conventional thin-film type magnetic recording media, wherein a fine-grained polycrystalline magnetic alloy serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetic domains of magnetic material. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate. Very high linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with such perpendicular magnetic media.
It is well-known that an efficient, high bit density perpendicular magnetic medium requires interposition of a relatively thick (as compared to the magnetic recording layer), magnetically “soft” underlayer or “keeper” layer, i.e., a magnetic layer having a relatively low coercivity below 1 kOe, such as Permalloy (a NiFe alloy, between the non-magnetic substrate (e.g., of glass, aluminum (Al) or an Al-based alloy), and the “hard” magnetic recording layer having a relatively high coercivity of several kOe, typically about 4–7 kOe (e.g., of a cobalt-based alloy such as CoCrPtB) having perpendicular anisotropy. The magnetically soft underlayer serves to guide magnetic flux emanating from the head through the hard, perpendicular magnetic recording layer. In addition, the magnetically soft underlayer reduces susceptibility of the medium to thermally-activated magnetization reversal by reducing the demagnetizing fields which lower the energy barrier that maintains the current state of magnetization.
A typical conventional perpendicular recording system 10 utilizing a vertically oriented magnetic medium 1 with a relatively thick soft magnetic underlayer, a relatively thin hard magnetic recording layer, and a single-pole head, is schematically illustrated in FIG. 1, wherein reference numerals 2, 3, 4, and 5, respectively, indicate a non-magnetic substrate, a soft magnetic underlayer, at least one non-magnetic interlayer, and a perpendicular hard magnetic recording layer. Reference numerals 7 and 8, respectively, indicate the single and auxiliary poles of a single-pole magnetic transducer head 6. The relatively thin interlayer 4 (also referred to as an “intermediate” layer), comprised of one or more layers of non-magnetic materials, serves to (1) prevent magnetic interaction between the soft underlayer 3 and the hard recording layer 5 and (2) promote desired microstructural and magnetic properties of the hard recording layer.
As shown by the arrows in the figure indicating the path of the magnetic flux φ, flux φ is seen as emanating from single pole 7 of single-pole magnetic transducer head 6, entering and passing through vertically oriented, hard magnetic recording layer 5 in the region below single pole 7, entering and travelling along soft magnetic underlayer 3 for a distance, and then exiting therefrom and passing through the perpendicular hard magnetic recording layer 5 in the region below auxiliary pole 8 of single-pole magnetic transducer head 6. The direction of movement of perpendicular magnetic medium 1 past transducer head 6 is indicated in the figure by the arrow above medium 1.
With continued reference to FIG. 1, vertical lines 9 indicate grain boundaries of each polycrystalline (i.e., granular) layer of the layer stack constituting medium 1. In the figure, the width of the grains (as measured in a horizontal direction) of each of the polycrystalline layers constituting the layer stack of the medium is shown as being substantially the same, i.e., each overlying layer may replicate the grain width of the underlying layer. A protective overcoat layer 11, such as of a diamond-like carbon (DLC) is formed over hard magnetic layer 5, and a lubricant topcoat layer 12, such as of a perfluoropolyethylene material, is formed over the protective overcoat layer. Substrate 2 is typically disk-shaped and comprised of a non-magnetic metal or alloy, e.g., Al or an Al-based alloy, such as Al—Mg having an Ni—P plating layer on the deposition surface thereof, or substrate 2 is comprised of a suitable glass, ceramic, glass-ceramic, polymeric material, or a composite or laminate of these materials. Underlayer 3 is typically comprised of an about 500 to about 4,000 Å thick layer of a soft magnetic material selected from the group consisting of Ni, NiFe (Permalloy), Co, CoZr, CoZrCr, CoZrNb, CoFe, Fe, FeN, FeSiAl, FeSiAlN, FeCoC, FeCoB, etc. Interlayer 4 typically comprises an up to about 300 Å thick layer of a non-magnetic material, such as TiCr. Hard magnetic layer 5 is typically comprised of an about 50 to about 250 Å thick layer of a Co-based alloy including one or more elements selected from the group consisting of Cr, Fe, Ta, Ti, Ni, Mo, Pt, V, Nb, Ge, B, Si, 0, and Pd; iron nitrides or oxides; or a (CoX/Pd or Pt)n multilayer magnetic superlattice structure, where n is an integer from about 10 to about 25, each of the alternating, thin layers of Co-based magnetic alloy is from about 2 to about 3.5 Å thick, X is an element selected from the group consisting of Cr, Ta, B, Mo, Pt, W, and Fe, and each of the alternating thin, non-magnetic layers of Pd or Pt is about 10 Å thick. Each type of hard magnetic recording layer material has perpendicular anisotropy arising from magneto-crystalline anisotropy (1st type) and/or interfacial anisotropy (2nd type).
Notwithstanding the improvement (i.e., increase) in areal recording density and SMNR afforded by perpendicular magnetic recording media as described supra, the escalating requirements for increased areal recording density, media stability and SMNR necessitate further improvement in media performance.
As indicated above, perpendicular magnetic recording media typically include a magnetically soft underlayer for guiding magnetic flux through the media and to enhance writability, a thin intermediate or interlayer, and a main recording layer. The role of the intermediate or interlayer (or multiple layer structure) is critical for obtaining good media performance. Specifically, in perpendicular magnetic recording media the intermediate or interlayer (or multiple layer interlayer structure) serves to provide:
1. control of the crystallographic orientation of the main recording layer;
2. control of the grain size and grain distribution of the main recording layer; and
3. physical separation between adjacent grains of the main recording layer, which feature is particularly desirable and important when the latter is formed by a low temperature and/or reactive sputtering process, so that growth of an oxide (e.g., Co-oxide or Si-oxide) occurs in the boundaries between adjacent grains.
More specifically, the SMNR of perpendicular magnetic recording media is improved by increasing the strength of the preferred c-axis out-of-plane orientation of the perpendicular main recording layer while maintaining a small uniform grain size of the layer. The preferred orientation of the magnetic layer depends upon the structural properties of and the interactions between the various previously deposited underlying layers of the media, as well as upon the nature of the substrate.
In general, control of the strength (or amount) of the preferred orientation of thin-film layers is difficult, for the reasons given above. Specifically, formation of a Co-alloy perpendicular magnetic recording layer with a strong <0002> growth orientation on a structure including a substrate, a soft magnetic underlayer, and non-magnetic intermediate or underlayer(s) between about 0.2 and 400 nm thick is extremely difficult.
As indicated supra, differences in crystallographic orientation between adjacent (e.g., lower and upper) thin film layers are caused by differences in the surface and interfacial energies of the materials of the layers, and by heteroepitaxial (or coherent) growth of one layer upon another layer of a chemically distinct material with different crystal lattice structure and atomic interplanar spacings.
The soft magnetic underlayer of perpendicular magnetic recording media is generally composed of a small grain or amorphous material containing at least one of Fe and Co. According to prior practice, a non-magnetic material of hcp structure, e.g., Ru, may be deposited on the soft magnetic underlayer, which non-magnetic hcp material grows with a moderately strong <0002> preferred orientation and small grain size. A magnetic material of hcp structure, typically a Co-based alloy, may grow coherently on the hcp non-magnetic layer, also with <0002> growth orientation and small grain size. The quality of the <0002> growth orientation can be determined from the size of symmetric X-ray diffraction peaks and X-ray rocking curves. Strong preferred growth orientation of the Co-based alloy with the hcp <0002> axis out-of-plane is generally necessary for achieving good performance of high areal recording density perpendicular magnetic media. Unfortunately, however, the quality of growth orientation of an hcp material upon the soft magnetic underlayer depends upon the selected material, and prior intermediate or underlayer structures, such as with a Ru layer and a Co-alloy layer, generally have not afforded the desired strength of <0002> growth orientation.
In view of the foregoing critical nature of the intermediate or interlayer in perpendicular magnetic recording media, there exists a clear need for improved layer structures for facilitating highly oriented crystal growth thereon and, more particularly, for highly oriented perpendicular magnetic recording media with enhanced performance characteristics, comprising improved intermediate or interlayer structures.
The present invention, therefore, addresses and solves problems attendant upon the design and manufacture of improved layer structures for facilitating highly oriented crystal growth and, in particular, fabrication of high performance, ultra-high areal recording density perpendicular magnetic recording media, while maintaining full compatibility with the economic requirements of cost-effective, large-scale automated manufacturing technology.